@teslas:

You can leave SZ 6 relatively undefended. The destroyer block between hawaii/japan goes without saying, and buys you one round every game

Just for clarity, you mean a single destroyer in SZ 16?

What is the typical US response to this? Do you find that it gets removed every turn (by air from Hawaii) and has to be replaced (so basically a sink of 8 IPC per turn)? Or is it typically ignored?

Long time lurker….

For this reason, I always buy both subs and destroyers in equal parts.  You have  the flexibility of a destroyer picket when you need it, or to provide some fodder from an air only attack, and you have the cheap subs to provide fodder for the massive fleet attack/engagement.

Sometimes, balance is the min/max…